1. Field of the Invention
This invention relates to a semiconductor device and a method for manufacturing the same, and more particularly to a semiconductor device and a method for manufacturing the same which are convenient in resin-sealing.
2. Description of the Prior Art
As the prior art of semiconductor packaging, thick film technique, thin film technique, resin sealing, bonding, etc. are known.
Then, for example, a semiconductor device using a so-called flat-pack type surface packaging will be described. Intended to increase packaging density on a printed circuit board, this is a type of method which narrows a distance between terminals (outer leads 4b as will be described later) projecting from a mold resin such as epoxy resin and puts out these terminals to the facing sides (or 4 sides) of the mold resin to stick them flatly to the printed circuit board.
An example shown in FIGS. 15 and 16 is a semiconductor device which has a packaging structure called QFP (quad flat package) among the aforementioned flat-pack type ones. FIG. 15 is a plane view of a lead frame showing a state after resin-sealing, and FIG. 16 is a plane view of a lead frame showing a state before resin sealing.
As shown in FIG. 16, the respective leads 4 (11 pieces in each of 4 directions, and totally 44 pieces in this example) provided over a lead frame 40 are composed of inner leads 4a and outer leads 4b. The inner leads 4a are electrically connected with a semiconductor element (for example, an IC chip containing dynamic RAM or the like) 7 mounted on a mount portion 10, by way of a bonding wire (for example, gold wire) 9 fixed to a pad (not shown) on the semiconductor element 7. Moreover, the inner leads 4a and the semiconductor element 7 are, as shown in FIG. 15, formed with a resin (for example, epoxy resin) 6 by way of a transfer mold.
Resin sealing is performed by sandwiching the lead frame 40 between the upper and lower molds of a mold for molding (not shown) to fix it, pouring the resin 6 into a cavity that produces the shape of a package 5, and then hardening said resin 6. However, if there are no dam bars 4c for restricting the outflow of sealing resin as will be described later, the lead frame 40 is, as mentioned above, sandwiched between the upper and lower molds (that is, the lead 4 has a thickness) when the lead frame 40 is fixed to the mold for molding, unavoidably creating spaces 3 between the respective adjacent leads 4. As a result, when the resin 6 is poured into the cavity of said mold, the resin 6 poured passes through the spaces 3 between the respective adjacent leads 4 and in turn flows out along the longitudinal direction of said outer leads 4b, thus causing a problem that no satisfactory resin sealing can be performed.
A conventional method commonly used to solve the above-mentioned problem is to provide dams 4c (which are so-called tie bars as shown in FIGS. 15 and 16) for restricting the outflow of mold resin. However, since these dam bars 4c are formed as a part of the lead frame 40 in the predetermined areas of the leads 4 (outer leads 4b), it is necessary after resin-sealing to cut and remove them and thereby separate the adjacent leads 4 (outer leads 4b). In other words, a process of cutting the dam bars 4c is required. And since a special jig is used to cut the dam bars 4c, the outer leads 4b may be given a damage such as unexpected deformation in cutting them. In addition, especially the lead pitch has recently become very narrow, for example, 0.3 mm, and thus made the cutting width of the dam bars 4c less than 0.3 mm, so that working with the aforementioned cutting jig has become very difficult.
Numerals 1 and 2, respectively, in FIGS. 15 and 16 are through-holes, which are to feed the lead frame 40 to each step of packaging process. In addition, 6a shows a resin portion resulting from outflow of the sealing resin 6 to the dam bars 4c and is removed when the above-mentioned dam bars 4c are cut.
Other than the above described method, there is another method of allowing the same effect as the above-mentioned dam bars 4c by providing dams to, for example, a mold for molding. This method is advantageous in that the dam bars 4c do not need to be cut, but if the aforementioned lead pitch becomes less than 0.3 mm, production of its mold becomes extremely difficult or virtually impossible.